Since the dawn of the petroleum industry, a continuous problem has been separation of oil and water. Nearly all crude oil produced in the world arrives at the earth's surface as a combination of oil and water. In some crude oil, water may be a rather minor component but in most it is a significant component. Further, separation of oil and water is made more difficult when the base fluid is an emulsion wherein water is in small droplets suspended in an oil base.
The basic mechanism by which water is separated from oil is by the use of gravity. Most oil production, after arriving at the earth's surface, is passed through a separator—that is, a vessel in which the crude oil is introduced. The vessel establishes a relative quiescence zone which allows the water to settle to the bottom and oil to rise to the top in the same way that cream rises in milk. Water is drawn from the bottom and oil from the top of the separator. With some crude oils, gravitational separation works efficiently but with other crude oil it is more difficult. It is apparent that if water and oil are not highly emulsified—that is, if water is not in the form of very small or even microscopic droplets, gravitational separation is effective. However, in many applications, water is so finely dispersed in an oil base that gravitational separation is not completely effective, in which case, additional treatment techniques are required.
One standard technique for improving the effectiveness of oil/water separation is by the use of coalescence. By various techniques, small water droplets suspended in oil can be caused to coalesce—that is, to join together to form larger water deposits. As water droplet size increases, the dynamics of gravitational separation improve—that is, large water droplets more freely fall out of an emulsion compared to small water droplets. Treating oil and water emulsions by coalescence is a technique that has long been employed in the petroleum industry.
A basic coalescence concept is to pass an emulsion through an established electric field. A typical way of establishing an electric field is to position spaced apart electrodes, normally metal plates, within the interior of a vessel located so that at least a portion of the emulsion passes between them as the emulsion moves through the vessel. Some vessels constructed to augment separation by electrostatic coalescence have a single input and a single output so that no actual separation occurs with the vessel. Such equipment for coalescing can be used in advance of another piece of equipment wherein actual separation of water and oil takes place. For instance, an electrostatic coalescer in which no separation takes place can be used in advance of a hydrocyclone, sometimes referred to as a vortex tube. The emulsion is treated by subjection to an electric field to augment the size of water droplets prior to passage of the emulsion into the hydrocyclone so that the increased size droplets are more efficiently separated by cyclonic action. The same piece of equipment can be used for passing an emulsion through an electric field before conveying the emulsion into a separation vessel when separation takes place by gravity. However, the most frequently employed equipment for treating an emulsion with an electric field is to provide spaced apart plates within a vessel in which the vessel has an emulsion inlet, an upper lighter component (oil) outlet and a lower heavier component (water) outlet. In this way, coalescence and separation are achieved in the same vessel.
A typical system for coalescing heavier and lighter components of an emulsion is illustrated in U.S. Pat. No. 4,400,253 entitled: “Voltage Control System for Electrostatic Oil Treater”, having issued on Aug. 23, 1983. In this disclosure, the electric field intensity increases then decreases periodically to augment coalescence. U.S. Pat. No. 4,417,971 entitled: “Circuit for Maintaining the Strength of an Electrostatic Field Generated in a Fluid Mixture of Varying Dielectric Strength” issued on Nov. 29, 1983 teaches a system of enhancing coalescence using electric fields in which rectifiers are arranged to maintain the strength of an electrostatic field as the dielectric strength of the liquid mixture changes.
The present invention is a method and a system of augmenting the separation of immiscible heavier and lighter components of an emulsion including the steps of introducing the emulsion into a vessel having an electric field therein to which the emulsion is subjected while varying the electric field at a frequency F1 modulated in amplitude at a frequency F2 wherein F1 is greater than F2.
Additional background information relating to the separation of heavier and lighter components of an emulsion can be obtained from the following United States patents:
PATENT NO.INVENTORTITLE1,116,299Laird et al.Process of Treating PetroleumEmulsions1,276,387McKibbenMethod of SeparatingAssociated Liquids2,120,932DillonHigh Frequency InductionDehydrator2,849,395WintermuteMethod and Apparatus forElectrical Separation ofEmulsions3,772,180PrestridgeElectric Treater3,839,176McCoy et al.Method and Apparatus forRemoving Contaminants fromLiquids3,847,775PrestridgeProcess for ElectricalCoalescing of Water4,126,537PrestridgeMethod and Apparatus forSeparation of Fluids with anElectric Field4,161,439Warren et al.Apparatus for application ofElectrostatic Fields to Mixingand Separating Fluids4,200,516PopeElectrostatic Coalescing System4,204,934Warren et al.Process for Application ofElectrostatic Fields to Mixingand Separating Fluids4,224,124PopeElectrostatic Coalescing System4,283,290DaviesPurification Utilizing LiquidMembrane with ElectrostaticCoalescence4,290,882DempseyElectrostatic Separation ofImpurities Phase from Liquid-Liquid Extraction4,308,127Prestridge et al.Separation of Emulsions withElectric Field4,400,253Prestridge et al.Voltage Control System forElectrostatic Oil Treater4,415,426Hsu et al.Electrodes for ElectricalCoalescense of LiquidEmulsions4,417,971Ferrin et al.Circuit for Maintaining theStrength of an ElectrostaticField Generated in a FluidMixture of Varying DielectricStrength4,469,582Sublette et al.Electrically Enhanced InclinedPlate Separator4,479,164SiegelControl for an ElectrostaticTreater4,581,119Rajani et al.Apparatus for Separating aDispersed Liquid Phase from aContinuous Liquid Phase byElectrostatic Coalescence4,581,120SubletteMethod and Apparatus forSeparating Oilfield Emulsions4,601,834Bailes et al.Settling of Liquid Dispersions4,606,801Prestridge et al.Electrostatic Mixer/Separator4,702,815Prestridge et al.Distributed ChargeComposition Electrodes andDesalting System4,747,921Bailes et al.Liquid-Liquid Contacting4,767,515Scott et al.Surface Area Generation andDroplet Size Control in SolventExtraction Systems UtilizingHigh Intensity Electric Fields4,804,453Sublette et al.Resolution of Emulsions withMultiple Electric Fields5,147,045Chi et al.Particulate Separations byElectrostatic Coalescence5,411,651Yamaguchi et al.Method for ElectrostaticLiquid/Liquid Contractor5,421,972Hickey et al.Process and Apparatus forRemoving SolubleContaminants fromHydrocarbon Streams5,464,522MacEdmondsonElectrostatic Oil Emulsion andTreating Method and Apparatus5,543,027Yamaguchi et al.Apparatus for ElectrostaticLiquid/Liquid Contactor5,565,078Sams et al.Apparatus for Augmenting theCoalescence of Water in aWater-In-Oil Emulsion5,575,896Sams et al.Method and Apparatus forOil/Water Separation Using aDual Electrode CentrifugalCoalescer5,643,431Sams et al.Method for Augmenting theCoalescence of Water In AWater-In-Oil Emulsion5,824,203RemoMethod and Means forChanging Characteristics ofSubstances6,010,634Sams et al.System and Method ForSeparating Mingled HeavierAnd Lighter Components Of ALiquid Stream6,113,765Wagner et al.Methods for EnhancedResolution of HydrocarbonContinuous Emulsions orDispersions with ConductivityModifiersGB 1,205,562Thornton et al.Liquid/Fluid Extraction Process